1. Field of the Invention
The present invention relates to a light unit for a display device, which is used as a back light of a liquid crystal display(LCD) panel and the like, and more particularly to a light unit for a display device, which is adapted to allow light entered thereinto from the side to exit in a direction perpendicular to the light unit.
2. Description of the Related Art
Currently, a liquid crystal display device(LCD) is widely used for monitors of personal computer monitors, flat televisions, or cellular phones, and the like. The liquid crystal display device requires a separate light source since it is not an independently light-generating device. Therefore, the liquid crystal display device is provided with a planar type light source, namely, a back light. Such a back light is configured to convert a linear shape light source, such as a cold cathode discharge tube, into such a planar type light source.
Considering various methods for constructing a light unit more specifically, there is one method for installing a light source under the rear surface of liquid crystal elements. Another method takes a side light manner for installing the light source at one side of the light guide plate of a light-transmitting type, such as an acrylic plate, and converting light emitting from the light source into planar light rays using the light guide plate, thereby achieving such a planar type light source. According to a further method, optical elements consisting of a prism array, and the like are installed at the light exiting surface of the light unit so as to achieve a desired optical property.
Among the above various methods, the method using the light guide plate installed at one side of the light source may utilize a multi-faceted prism 15 as shown in FIG. 1. FIG. 1 is a view illustrating a light unit of the prior art using such a prism.
As shown in FIG. 1, a light unit of the prior art comprises a light guide plate 13, a light source 10 installed at one side of the light guide plate 13 of a white light emitting type, a reflector plate 11 located under the light guide plate 13, and a diffuser sheet 14, multi-faceted prisms 15 and protective sheet 16 arranged in order above the light guide plate 13. Additionally, above the protective sheet 16, a liquid crystal display(LCD) panel 17 is disposed. The light guide plate 13 has a printed dot pattern or a V-shape groove pattern 18, as a dispersion pattern, at the lower portion thereof.
In operation, white light emitted from the light source 10 enters the light guide plate 13. After entered, the light exits from the light guide plate 13 at an angle at which it overcomes total reflection by virtue of the dispersion pattern formed at the light guide plate 13. After that, the exiting light becomes uniform in brightness by the diffuser sheet 14, and the prisms 15 gather the light to emit it frontward.
The light source 10 emits the white light of multi-wavelength, and the light guide plate 13 is formed at its upper portion with a certain diffraction pattern. Thus, when the white light enters the light guide plate 13, the incident rays of white light, such as Red, Green and Blue according to wavelength, are diffracted by the diffraction pattern of the light guide plate 13, thereby exiting from the light guide plate 13 at different diffraction angles according to wavelength. Due to a difference between the diffraction angles according to wavelength, the light is dispersed when it exits frontward, thereby causing a separation problem of white light into its component colors. This color separation phenomenon is well shown in FIG. 2. As shown in FIG. 2, light exiting angles of the rays R, G and B are not coincident with each other.
In order to solve the above problem, the light unit of the prior art shown in FIG. 1 adopts the diffuser sheet 14. Such a diffuser sheet 14 serves to increase the brightness uniformity of light so as to produce planar light rays of a uniform brightness. The diffuser sheet 14 is formed by randomly processing the surface of a film so that it can disperse light entered thereinto. That is, the diffuser sheet 14 is used to diffuse the light entered thereinto, thereby attenuating the color separation phenomenon of the light caused when the rays of different wavelengths exit at different angles.
The diffuser sheet 14 as described above, however, only functions to spread the light, thereby simply causing the rays of different wavelengths to overlap with each other. The diffuser sheet 14 cannot completely eliminate the color separation phenomenon of the light due to the different exit angles according to wavelength.
Further, since the light unit shown in FIG. 1 comprises the plural prisms and the diffuser sheet, the whole structure of the light unit is complex.
FIG. 3 is a view illustrating another light unit of the prior art, using a hologram pattern. Referring to FIG. 3, light emitting from a side light source 20 enters a light guide plate 21. The light guide plate 21 is formed with a hologram diffraction pattern 22. Such a hologram diffraction pattern 22 serves to cause the light proceeding through the light guide plate 21 to exit frontward from the light guide plate 21 at an angle of about 90°.
The light unit not using a prism as shown in FIG. 3 also suffers from the color separation phenomenon, similar to the case of FIG. 2. The light, to exit frontward from the light guide plate 21, is diffracted by the hologram diffraction pattern 22 formed at the upper portion of the light guide plate 21. As a result, the respective rays of light according to wavelength exit at different diffraction angles, thereby causing the exiting light to be dispersed into its several component colors according to wavelength.
Therefore, the light unit using the hologram diffraction pattern as shown in FIG. 3 requires a pattern design as shown in FIG. 4. FIG. 4 is a view illustrating a patterned surface of a light guide plate. The patterned surface comprises various different diffraction patterns corresponding to the respective wavelengths of light exiting from the light guide plate. That is, the patterned surface is divided into three kinds of regions for allowing the respective wavelengths R, G and B to exit in a direction perpendicular to the light guide plate, respectively. In this case, each of the regions is adapted to allow only one specific wavelength R, G or B to exit in a direction substantially perpendicular to the light guide plate, and these regions are continuously arranged, thereby preventing the user from recognizing the color separation phenomenon.
Such a continuous arrangement of different patterns on the light guide plate as stated above, however, complicates a manufacturing process, thereby deteriorating productivity. Furthermore, this arrangement of the hologram pattern is not an essential solution to the color separation problem.